Clinical evaluation of cochlear implant sound coding taking into account conjectural masking functions, MP3000™

Abstract

Efficacy of the SPEAK and ACE coding strategies was compared with that of a new strategy, MP3000™, by 37 European implant centers including 221 subjects. The SPEAK and ACE strategies are based on selection of 8–10 spectral components with the highest levels, while MP3000 is based on the selection of only 4–6 components, with the highest levels relative to an estimate of the spread of masking. The pulse rate per component was fixed. No significant difference was found for the speech scores and for coding preference between the SPEAK/ACE and MP3000 strategies. Battery life was 24% longer for the MP3000 strategy. With MP3000 the best results were found for a selection of six components. In addition, the best results were found for a masking function with a low-frequency slope of 50 dB/Bark and a high-frequency slope of 37 dB/Bark (50/37) as compared to the other combinations examined of 40/30 and 20/15 dB/Bark. The best results found for the steepest slopes do not seem to agree with current estimates of the spread of masking in electrical stimulation. Future research might reveal if performance with respect to SPEAK/ACE can be enhanced by increasing the number of channels in MP3000 beyond 4–6 and it should shed more light on the optimum steepness of the slopes of the masking functions applied in MP3000.

Keywords:

• Cochlear implant
• Speech coding
• Masking
• Spread of excitation
• Battery life

Acknowledgements

The authors would like to express their gratitude to the following contributors: John Allum, Roberto Bartoli, Elisabeth Estrada, Juan Carlos Falcón González, Stefan Gräbel, Heinrich Iro, WaiKong Lai, Roland Laszig, Manuel Manrique, Andreas Nägeli, Erwin Offeciers, Cathy Silhol, Guillermina Tarayre, Alain Uziel, and Simone Volpert.

Glossary

ACE: advanced combinational encoder, similar to SPEAK but with higher pulse rates

CIS: continuous interleaved sampling, a strategy in which a fixed number of frequency channels is sampled continuously in a fixed order

MP3000: a coding strategy in which only N channels of M available channels are chosen dynamically according to the instantaneous peak levels in all channels relative to a physiologically-based masking function

N-of-M: a coding strategy in which only N channels of M available channels are chosen dynamically

SPEAK: spectral peak strategy, a strategy in which only N channels of M available channels are chosen dynamically according to the instantaneous peak levels in all channels